NOTES AND EXTENSIONS. APPENDIX I, (SECOND EDITIONT.) CHAPTER. I. P. j/. Brass and Gun-metal Founding.—With these alloys a good height of pouring head is required, so as to cause pressure and prevent porosity ia the casting. An ample number of risers should also be provided to permit escape of air or gas, and thus avoid honey-combing. No blackening is used, but the mould is faced with very fine sand. JP. 42. Steel Casting.—It having been noted that small castings were more porous than large ones, the conclusion was arrived at that it was of the utmost importance to keep the metal at a very great heat until poured. The present method of pre- venting blow-holes is to add silica-Spiegel (a combination of silicon and pure cast iron) to the ladle while pouring, the better plan being to apply it ia the molten condition. If this substance be not thoroughly mixed we get 'hard' and 'soft' spots in the casting, the former being due to accumulations of the silicon. In order to keep the casting uniformly hot until the whole mould be filled, the pouring gate should be chosen at the heaviest por- tion of the casting; and small castings are preferably poured from a small converfer to themselves, instead of from the refuse of a large open-hearth melting. By facing the mould, where required, witli ferro-raanganese or ferro-chromium, very hard surfaces are obtained in those places, the latter substance giving the hardest result.